I am trying to run this code but getting an error:

My code:
class Student:
def __init__(self, student_id, marks,age):
self.student_id=student_id
self.marks=marks
self.age=age
def validate_age(self):
if self.age >20:
return True
else:
return False
def validate_marks(self):
if self.marks>0 and self.marks<100:
return True
else:
return False
def check_qualification(self):
if (validate_age)==True and if(validate_marks)==True:
if self.marks>65:
return True
else:
return False
def set(self,x):
self.__student_id=x
def get(self):
if (check_qualification==True):
return True
else:
return False